For decades, doctors thought that bacteria did not grow in healthy people’s lungs. But an emerging body of research, including a new paper from Stanford and Lucile Packard Children’s Hospital, shows that the received wisdom is wrong. Far from being sterile, the study shows, healthy lungs are home to a veritable forest of microbes.

The paper, which appears today in Science Translational Medicine, compares the microorganisms in the lungs of healthy people and cystic fibrosis (CF) patients. CF is a genetic disease that causes severe, progressive lung dysfunction and death from respiratory failure. Healthy people have far more diversity in their lung microorganisms than those with CF and the mix of lung bacteria differs substantially between the two groups, researchers found.

For example, the researchers confirmed the presence of Pseudomonas aeruginosa (shown in the colorized scanning electron micrograph image above) in the lungs of CF patients, where it is frequently associated with pneumonia. In contrast to this well-known pathogen, a large proportion of the microbial residents in healthy people’s lungs have never been grown in a lab.

Researcher David Cornfield, MD, a Packard Children’s pulmonologist who treats patients at the Cystic Fibrosis Center at Stanford, likens the lung to a rainforest, speculating that the decreased diversity in the CF lung’s microbes may be just as harmful as allowing one species to take over in a delicately balanced ecosystem of a rainforest.

As Cornfield explained in our release, the findings open a wide array of possibilities for novel avenues in the treatment of CF and other lung diseases:

“We may need to consider strategies that allow favorable microbes to exist while eradicating disease-causing species,” Cornfield said. “That paradigm, if it’s true, would really turn the care of patients with pneumonia and other lower-airway diseases on its head.” Future research might test whether CF or pneumonia patients could benefit from doses of probiotic bacteria to their lungs, he said.